The Multi-Decadal Response to Net Zero CO2 Emissions and Implications for Emissions Policy

Abstract

How confident are we that CO2 emissions must reach net zero or below to halt CO2-induced warming? The IPCC's sixth assessment report concluded that “limiting human-induced global warming to a specific level requires … reaching at least net zero CO2 emissions.” This is much stronger language than the special report on the global warming of 1.5°C, which concluded that reaching net zero CO2 emissions would be sufficient. Here we show that “approximately net zero” is better supported than “at least net zero.” We estimate the rate of adjustment to zero emissions (RAZE) parameter (−0.24 to +0.17%/yr), defined as the fractional change in CO2-induced warming after CO2 emissions cease. The RAZE determines the CO2 emissions compatible with halting warming over multiple decades: in 1.5°C-consistent scenarios, CO2 emissions consistent with halting anthropogenic warming are +2.2 GtCO2/yr (5–95th percentile range spans −7.3 to +6.2 GtCO2/yr), similar to the expected emissions from unmodelled Earth system feedbacks.